Differences in the Bills of Sparrows on Islands Is Driven by Climate

January 1, 2013 by Russell Greenberg and Raymond Danner

The bird bill is the knife, fork, and spoon for dining birds. In most cases, male and female birds come prepared to dine with the same cutlery. Every once in a while the two sexes diverge in their bill size and shape. This seems to be particularly true of islands. The traditional thinking is that the bill is honed by natural selection for birds to best forage for and handle the food they eat. Competition between species enforces species-specific specialization. On islands, this interspecific competition may be reduced and the sexes can feed on different foods to reduce within-pair competition. This may be particularly important as the pair strains to provide day-long meals to their hungry broods. As in "honey, can you pick up some milk, I am heading off to buy macaroni and cheese."

As compelling as this story is, it lacks a large amount of empirical support. Recent work has begun to focus on alternative functions for the bird bill, which includes the dissipation of excess heat. To apply this to mom and dad birds, the male’s reproductive (and hence evolutionary) success in most songbirds, depends on patrolling a territory from exposed sites into the heat of the day. Females are free to skulk in the shade. Thus, the pair members have different solar-thermal niches and different requirements for heat loss. Could this also affect bill size and shape?

With all of this in mind, we measured thousands of song sparrows (Melospiza melodia) collected last century throughout California and the California islands. We found that bill size increased at higher summer temperatures, supporting thermal role for the sparrow bill. But more interestingly, we found that bill size was distinctly different between male and female island sparrows and that male sparrow bills are particularly large on the warmer islands.

These observations support the idea that island life allows more divergence in bill size between the sexes, but that climate—particularly heat during warm, dry Mediterranean summers of California, is the evolutionary driver of sexual bill dimorphism. We plan to keep our eye on this and see how climate warming, particularly off the coast of Southern California might have affected bill size change in the past and into the future.